This invention relates to the formation of sulfur from hydrogen sulfide, and in particular to the recovery of valuable salts from the Stretford process or the like.
In the so-called Stretford process, an alkaline solution with salts of anthraquinone disulfonic acid (ADA) and a vanadate (+5) is employed as the absorbing liquid (or scrubbing liquid) to absorb hydrogen sulfide from gases or gaseous mixtures containing the hydrogen sulfide. (In this specification and the appended claims, the terms absorber and scrubbing column, and absorbing liquid and scrubbing liquid, respectively, are used interchangeably to designate identical unit operations and process streams, respectively.) The alkaline component is usually sodium carbonate. To accelerate certain partial reactions, sodium vanadate is added in a more modern version of the Stretford process. In the first reaction step, which takes place at a relatively high speed, sodium hydrogen sulfide (sodium bisulfide) is formed in a reaction of hydrogen sulfide with sodium carbonate. Subsequently, elemental sulfur is obtained in a reaction of the sodium hydrogen sulfide with sodium vanadate with a simultaneous reduction of vanadium from +5 to +4 and with the formation of sodium hydroxide solution. The presence of the salt of anthraquinone disulfonic acid is necessary in this process to oxidize the vanadium to its original oxidation state (i.e., +5) with the sodium hydroxide solution. The salt of anthraquinone disulfonic acid, reduced in this process step, is later reoxidized by the addition of oxygen, usually introduced as a stream of air. During this regeneration of the absorbing liquid, elemental sulfur is recovered by flotation during the addition of oxygen (e.g., air blowing).
Aside from producing sulfur in the Stretford process, undesired by-products are formed.
These by-products are sodium thiosulfate and sodium sulfate. A certain percentage of these by-products in the absorbing liquid, which, following the regeneration, is repeatedly recirculated, can be tolerated without adverse effects on the process; however, when a certain limit value is exceeded, corrosion and precipitates occur leading to a troublesome clogging of the apparatus.
To prevent such clogging, it has been proposed [DOS (German Unexamined Laid-Open Application) No. 2,364,267 which is incorporated herein by reference] to continuously branch off a side stream of the absorbing liquid and subject same to a combustion or high-temperature hydrolysis in a non-oxidizing atmosphere. During this step, a solid residue is formed containing the undesirable salts, primarily in the form of sodium sulfate. Gaseous hydrogen sulfide obtained during the hydrolysis is combined with the raw feed gas mixture to be introduced into the hydrogen sulfide absorption stage. The main disadvantage of this prior art process is that the salts of anthraquinone disulfonic acid contained in the side stream of the absorbing liquid are also decomposed during the hydrolysis to form carbon oxides, sodium sulfate, and water. This leads to a continuous destruction of the relatively expensive salts of anthraquinone disulfonic acid.